Aramid fiber is also called “liquid crystal nylon†or Kevlar fiber, and its scientific name is “polyphenylene terephthalate (PPTA). The main varieties of aramid fiber are polyparaphenylene terephthalamide fiber and poly-m-phenylene. Dimethyl phenylenediamine fiber has broad prospects for use in aviation, rocket and high performance ships due to its ideal hardness.
1 Physical and chemical properties The main chain of the aramid macromolecule consists of an aromatic ring and an amide bond, and at least 85% of the amide groups are directly bonded to the aromatic ring, and the nitrogen atom and the carbonyl group in the amide group of each repeating unit are directly It is connected to a carbon atom in the aromatic ring and replaces one of the hydrogen atoms. The structure is an extended linear, highly oriented, highly crystalline organic polymer material, the conformation of the macromolecular chain is approximately linear, the cross-sectional area of ​​the macromolecular chain is small, and the angular deformation of the polymer chain and the resistance of the rotation within the bond are large. High relative molecular mass.
Aramid fibers have high specific strength, specific modulus and stress cracking, but have poor compression, shear and wear properties.
Aramid fiber is resistant to the attack of general chemicals. Most organic solvents have little effect on its strength. Most aqueous solutions of salt have no effect on it, but strong acid and alkali will erode aramid at high temperature or high concentration.
Aramid fibers are sensitive to UV light and must be kept away from harmful UV light or away from glass windows when stored.
Aramid fiber has good thermal stability and maintains various properties at 180 °C. Since the aramid fiber does not melt and does not support combustion, it is exposed to 300 ° C or more for a short period of time, and has little effect on the strength. It does not become brittle at a low temperature of 17012, and the performance can be maintained.
Due to its high crystallinity and unidirectionality, aramid fiber has very low creep. In addition to the magnitude of stress and temperature, creep can also increase creep if it is soaked in water. Between 20 ° C and 150 ° C, if the stress is less than 50% of the tensile strength, the creep rate of the aramid does not change due to the magnitude of the stress and the temperature. If the stress is higher than 70% of the tensile strength, the creep rate Will accelerate faster.
The tensile strength of aramid fiber is 5 times larger than that of copper wire and 2 times larger than that of glass wire. Aramid fiber has a low density, about 20% lighter than carbon fiber, 40% lighter than glass fiber, low linear expansion coefficient and high elongation at break, excellent impact strength and high specific modulus of elasticity.
2 Process development Aramid fiber, or aramid fiber, is a synthetic fiber obtained by polycondensation spinning from an aromatic compound. Aramids include aramid fibers and aromatic polyimide fibers. The aramid generally mentioned is mainly an aramid fiber, which contains Kevlar fiber, Twaron (poly-p-phenylene terephthalamide fiber) from the Netherlands, and Conex (poly-m-phenylene diene) from Teijin, Japan. Phenylenediamine fiber), Technora (polyparaphenylene terephthalamide fiber), Russia's Teflon and China's aramid 1414. Among them, Kevlar fiber is the earliest commercialized and widely used product. The most variety of products.
Aramid II (aramid 1414) is obtained by polycondensation of p-phenylenediamine and terephthaloyl chloride in a low temperature solution to prepare a powdery polymer (aramid II resin), which is dissolved in concentrated sulfuric acid to form liquid crystal spinning. The silk liquid was further subjected to fiber formation by a wet spinning method.
The synthesis methods of PPTA include various methods such as low temperature solution polycondensation, gas phase polycondensation, high temperature catalytic polycondensation, interfacial polycondensation and direct polycondensation. Among them, the polycondensation reaction of the low temperature solution is the most mature, and industrial production has been realized. The optimum solvent for the component is a mixed solvent of N-methylpyrrolidone (NMP) and hexamethylphosphoramide (HMTA) or a metal salt such as I iCl or CaCl 2 .
The p-aminobenzoic acid was used as a raw material by a polycondensation method, and the polycondensation was carried out by heating in an NMP solvent. The polycondensation product was washed with water and dried to obtain aramid 1414 resin. The resin is dissolved in sulfuric acid or N-methylpyrrolidone to prepare a liquid crystal slurry, which is subjected to wet spinning and heat treatment to obtain aramid 1414 fiber having high strength and high modulus.
An equimolar ratio of high purity terephthaloyl chloride and p-phenylenediamine, a strong polar solvent (such as NMP, etc.) and other additives are added to the reactor, and precipitation polycondensation is carried out at a low temperature. The reaction is characterized by a fast reaction rate, a high degree of symmetry and regularity of the resulting polymer, and extremely difficult to dissolve in the solvent, and the polymer is isolated at the beginning of the reaction. In order to produce a high molecular weight product, the reaction apparatus must be provided with a strong stirring device or a structure having a strong shearing and kneading action. Therefore, it is more effective to use a twin-screw reactor with sufficient shearing action. The polymer containing a large amount of solvent and inorganic salt formed by the reaction needs to be subjected to solvent removal treatment and washed with ion-free water and dried to obtain a product.
Aramid 1414 is mainly used for spinning. It is dissolved in 98% or more of sulfuric acid, and can be obtained by spinning, washing and high-temperature treatment to obtain high-strength and high-modulus fibers.
PPTA has a very good chemical resistance (except for strong alkali acids or at high temperatures). The absolute thermal conductivity of PPTA is significantly lower than that of polyethylene fibers, and the coefficient of thermal expansion is also small, 1.5 times smaller than polyethylene. ~3 times, the rate of polyphenylene diamine and terephthaloyl chloride solution is very fast, because terephthalic acid chloride has high chemical activity and low reaction activation energy, and the activation energy of amide bond formation is only 6.28kJ/ The melting activation energy of the amide bond in the formed molecular chain is high, so the reaction rate constant of both can reach 106 mol/I •S. Under rapid agitation, the degree of polymerization can reach 1001 in the first few seconds of the polycondensation reaction. When the molecular weight reaches a certain high value (the viscosity of the system increases), the reaction rate decreases, the diffusion rate of the reactant decreases, and the system forms a gel. It takes a few minutes to rise from the gel to the desired polymer (molecular weight 123,000). The specific operation examples are as follows:
Example 1 After adding 150 g of NMP containing 5.5% anhydrous calcium chloride to a three-neck reactor equipped with a stirrer, a thermometer, and a nitrogen introduction tube, 0.1 mol of p-phenylenediamine was added under stirring, while stirring. After completely dissolving, the temperature of the liquid is lowered to 4I by using a cold water bath; equimolar terephthaloyl chloride is added, the material is quickly dissolved under strong stirring, and the reaction liquid is gradually thickened to form a condensed polymer, so that it cannot be Stirring was continued, and the mixture was allowed to stand overnight. The polymer was taken out, crushed, washed, and dried to obtain a finished product.
Example 2 After adding a solution of potassium t-butoxide and dimethyl sulfoxide to a three-neck reactor equipped with a thermometer, a stirrer and a nitrogen introduction tube, the liquid temperature was heated to 65 ° C with stirring, and then nitrogen gas was introduced under stirring. After 45 min, the reaction was cooled to room temperature and PPTA was added. A polyanion solution was formed after stirring for 48 h at room temperature. Different concentrations of polyanion solution can be obtained according to different ratios. This approach opens up new ways of processing for film and fiber processing.
Example 3 To a three-neck reactor equipped with a stirring, a thermometer, and a nitrogen gas line, 1 g of sodium hydride and 100 ml of dimethyl sulfoxide were added, and the temperature was heated to 70 ° C over a period of 29 minutes. The reaction was stirred for 1 h under nitrogen. After the reaction was completed, an equimolar amount of PPTA with sodium hydride was added. The reaction was carried out at reflux temperature for 4 h, and then ethylene oxide was added to continue the reaction until the solution was red. Finally, the reaction was terminated with a small amount of dilute acid to precipitate a high-solid composite PPTA product.
Poly(p-phenylene terephthalamide) is a polymer formed by condensation reaction of phenyldiamine and terephthalic acid at a low temperature. The polymer was filtered off, washed and dried, and spun in a strong acid solution by a dry jet spinning method. The filaments were extruded from a spinning nozzle at a temperature of 351 °C. After passing through the air layer, it is cooled and formed into 0-40C cold water. Finally, the tow is washed and then wound into a cake, and after drying, it becomes a PPTA product.
Example 4 Dissolving p-phenylenediamine in dimethylacetamide MA/LiCl solvent or hexamethylphosphoryl HMPA/NMP solvent may also be combined with solvent NMP/cacl2. Under the protection of nitrogen, while maintaining a temperature of 15-15 ° C, the terephthalic acid chloride (TPC) powder was quickly added, and the reaction temperature was kept at 25 ° C under vigorous stirring. The use of the amide solvent was beneficial to remove the hydrochloric acid formed by the polycondensation reaction. The reaction to keep the temperature low is to reduce side reactions. In order to obtain high molecular weight PPTA, it is necessary to use refined high purity monomer raw materials, p-phenylenediamine (purity greater than 99.5%), terephthaloyl chloride (purity greater than 92.5%) and low water content (water content) Less than 0.05%). Since the final product PPTA is a solid, it is necessary to separate and dry the PPTA from the solvent system, and then to form a liquid using liquid crystal spinning.
The two-step aramid spinning process has the disadvantages of complicated process, high production cost and high requirements on equipment materials. In order to shorten the process and simplify the process, one-step new process for directly spinning the fiber from the NMP solution of the polymer is explored. .
The aramid produced by the one-step process has superior properties to the aramid produced by the two-step process. For example, the acid resistance of aramid produced by Hossert in one-step process is far superior to that of conventional PPTA fiber, and its hydrolysis resistance is extremely excellent. The directly prepared aramid staple fiber has a large specific surface area and a relatively suitable aspect ratio, and is more suitable for composite materials. Also using the one-step method are the Japanese Teijin Company and the Japanese Asahi Kasei Corporation.
Due to the requirements of aerospace (high strength, high temperature resistance, etc.), DuPont has developed a well-known liquid crystal spinning method based on long-term research to make PPTA fibers directly spun into rigid polymers with poor spinnability. It is a unique and novel special spinning technology that opens up a new field of synthetic fiber textile engineering with new technology. Since the PPTA polymer is difficult or impossible to be dissolved by an organic solvent, the aromatic polyamide is dissolved into a liquid crystal solution using a salt-containing industrial sulfuric acid, and after being extruded from a spinning hole (generally, the fiber must be subjected to stretching treatment), without stretching, directly The PPTA29 fiber with high strength, high modulus and high temperature resistance is formed and further heat treated to form PPTA49 fiber. In high speed spinning, the speed of (dry-wet method) has exceeded 2000m/min, and the PPTA fiber structure is similar to that of rigid straight chain. Reticulated crosslinked crystalline polymer. The synthetic starting monomers used were p-phenylenediamine (PDA) and terephthaloyl chloride (TPC).
2.1 Copolymer fiber Aramid is a rigid molecule with high molecular symmetry, high degree of orientation and high crystallinity, so the lateral inter-molecular interaction becomes weak; there are a large number of aromatic rings in the molecular structure, which makes the macromolecular structure stable, and the intermolecular hydrogen The bond is weak, the reactive groups are few, the surface polarity is low, the lateral strength is low, and the fracture is easy under the action of compression and shearing force; and the surface of the aramid fiber is easy to absorb water, resulting in the combination of the aramid fiber and the resin matrix. The two-phase interface has low strength and small interlaminar shear strength, which affects the comprehensive performance of the composite material. Therefore, in order to give full play to the excellent mechanical properties of aramid, the modification of the surface of aramid and the improvement of the interfacial bonding of aramid reinforced composites have become a hot topic in the study of material mechanics: the use of a variety of aromatic or heterocyclic rings The copolymerization of amines and diacid chlorides with terephthaloyl chloride and p-phenylenediamine gives some properties of better fibers.
DuPont's ultra-high-strength ultra-high-modulus fiber manufacturing technology is to wash and neutralize the polyparaphenylene terephthalamide precursor obtained by dry-jet wet spinning, and then perform one or more microwave resonances in a wet state. Frequency heat treatment, temperature 270 ~ 310 ° C, frequency 915MHz, can produce strength up to 23.6cN / dtex, elongation of 2.86%, modulus of 706cN / dtex (or modulus of 1048cN / dtex, intensity of 19 .3cN/dtex), fiber with an elongation of 1.65%.
Russia's PPTA fiber is a copolymer fiber with a small amount of benzoxazole monomer. It has produced 5.0GPa high-strength wire, and the laboratory has produced ultra-high-strength wire of nearly 7.0GPa, which means this kind of future. Fiber may compete with the world's most advanced T-1000PAN-based carbon fiber produced by Toray. The former has superior specific strength and specific modulus, so it may develop into a high-tech product in the 21st century.
In the short aramid composite, the contribution of the fiber to the impact strength is multi-faceted: 1 the fiber is disconnected from the matrix and pulled out, the energy is dissipated due to friction, and the toughness is improved; 2 the fiber can hinder the crack propagation and the crack propagation When the fiber is perpendicular to the direction of the crack, it can be terminated by the fiber; 3 Because the aramid fiber has a high elongation at break, the fiber can absorb the impact energy after the impact; 4 the end of the fiber is the stress concentration zone. Stress concentration will also occur in the areas where the fibers are in contact with each other, and the impact strength is lowered. In aramid short fiber reinforced composites, the mechanical properties of the material are determined by the matrix, the fibers, and the interfacial properties between the fibers and the matrix.
Improving and optimizing the interfacial adhesion between the reinforcing material and the substrate is the most critical factor in the composite interface control technology. For the fiber reinforced composite material, it is mainly realized by modifying the surface of the reinforcing fiber. At present, the surface modification technology for aramid is mainly focused on improving the surface composition and structure of the fiber by chemical reaction, or improving the wettability between the aramid and the matrix resin by physical action.
The modification of para-aramid is also an important issue. The PPTA fiber of Teijin Company of Japan is directly spun, nascent fiber, by introducing a copolymerization component of a flexible group into the main chain to dissolve the copolymer in a polycondensation solvent. After high temperature and high magnification, high strength and high modulus fibers are obtained. The rigid and flexible monomer chain molecular design of copolymerized aramid has many kinds of diversity, and its rigid part is important for the high strength, high modulus and high crystallinity of the fiber, while the flexible link is related to the macromolecule in NMP. The solubility in the solvent and the thermal stretchability of the fiber have an effect on the elongation at break and the chemical resistance. Therefore, the choice of the flexible chain monomer and its ratio in the main chain are combined from the molecular design point of view and the experiment. Method to study. German Hoechst modified copolymer aramid has a performance strength of 2.8GPa, a modulus of 80GPa, an elongation of 4.6%, and a 1.5 times higher heat resistance and chemical resistance than para-aramid. The two techniques of plasma modification and chemical surface graft modification are obvious. The treatment results are that the active groups are introduced or generated on the surface of the fiber, and the interface is mainly composed of chemical bonds.
The surface of the plasma-treated aramid fiber can produce active groups such as -COOH, -OH, -COH, -C=O, -NH2, and become the main mechanism of interfacial chemical bond bonding and subvalent bond interaction. If plasma treatment with a monomer gas is used, a graft can also be formed on the surface of the aramid fiber. Although plasma treatment has many advantages, there is still a problem in the continuous treatment. The plasma modification technology needs to be carried out under strict vacuum during the treatment process, and the treated fiber should be compounded with the matrix as soon as possible to avoid surface activity. Degradation.
At present, surface modification technology is gradually developing from intermittent chemical modification to continuous, multi-angle online processing. Ultrasonic on-line treatment mainly enhances the wettability of aramid by reducing the viscosity and surface tension of the resin system, and the high-pressure forced resin impregnation of aramid by ultrasonic cavitation can greatly improve the wettability of the two. The initial wetting speed is increased by more than 90%, and the mechanical properties of the aramid reinforced composite can be improved after sonication. Processing methods with batch, continuous processing and easy industrialization are the main trends in surface technology research and development in the future.
2.2 Composite fiber or fiber assembly is formed into a whole through the resin matrix, so that the composite material has structural integrity, and the resin plays the role of transmitting load. Only the fiber or fiber assembly and the resin matrix can be matched and matched to fully exert the composite. The comprehensive properties of the material. The matrix has three main functions: 1 the matrix transmits the load to the fiber in the form of shear stress through its interface with the fiber; 2 protects the fiber from the chemical and physical damage of the external environment; 3 crack propagation preventing the fiber from breaking.
The choice of polymer resin matrix depends primarily on the performance requirements of the composite article. Factors that must be considered are cost, density, interfacial adhesion, tensile properties, compression and bending properties, resistance to solvents, moisture and temperature, and thermal expansion. Compatibility and workability, etc., usually low modulus or soft resin with high strain at break, good bending properties and strong adhesion, high modulus or rigid resin with high compression and brittleness And relatively weak bonding force.
The resin matrix of the aramid composite mainly includes epoxy resin (EP), unsaturated polyester (UP), vinyl ester, nylon, polyvinyl butyral, and polyethylene.
In the molding process of the fiber composite material, the matrix and the reinforcing fiber are combined into a specific shape by a certain physical and chemical change. When the surface-modified aramid fiber is compounded with the matrix, it is often necessary to add some modifier or interface design. The molding process of aramid composites is similar to that of general resin-based composites, mainly including: paste, molding, bag mold and RTM molding process.
The interface ultrasonic treatment system is applied to the interface treatment of the aramid/epoxy composite process, which greatly improves the interfacial shear strength, tensile strength and interlaminar shear strength, and the damage of the composite after ultrasonic modification. It is characterized by matrix cracking and microfibrillation of the surface of the fiber, and ultrasonic modification strengthens the interface of the composite.
3 Application development Aramid fiber has been favored as soon as it is published. Aramid fiber reinforced composites have excellent properties such as corrosion resistance, fatigue resistance, impact resistance and heat insulation, and have been widely used in many fields, especially in the aerospace industry.
Due to its excellent properties, aramid and its products are widely used, from military to civilian. In 1972, DuPont Company of the United States first introduced PPTA fiber, a high-performance material. The amide group on the molecular structure was separated by an aromatic ring and formed a II conjugation effect with the benzene ring. The internal rotation position was quite high, and the molecular chain was a plane rigid straight chain. . It has extremely high tensile strength (after glass fiber, graphite fiber and PBI fiber) and excellent heat resistance and toughness, and it also has good alkali resistance, organic solvent resistance and bleach resistance. As well as resistance to insects and mildew, therefore, although the original intention of developing aramid is for aerospace, it is now widely used in consumer textiles and industrial textiles, and the specifications of aramid are also diversified and differentiated.
3.1 Paper industry Aramid pulp fiber is a differentiated variety of aramid fiber, the chemical structure is the same as aramid, so it retains most of the excellent properties of aramid, such as: heat resistance, wear resistance, Dimensional stability and other properties, but due to its unique molding process, it has some characteristics that distinguish it from aramid long fibers. The density of aramid pulp is 1.41~1.42g/cm, slightly smaller than aramid, the surface is fluffy microfibres, the hairiness is rich, and its surface area is more than ten times that of fiber. The surface amino group content is also more than ten times that of the fiber, so that it has a good affinity with the amide-based composite resin, and can also form a hydrogen bond with some resins at the interface of the pulp, which can enhance the composite effect. Aramid does not pose a significant health hazard to producers and users. Aramid pulp fibers are widely used in industrial papermaking because of their excellent wet processing properties and enhanced properties.
The pulp is easily dispersed in paper, and the aspect ratio of the pulp fibrils provides the strength of the aramid pulp. In addition to improving the tensile strength, modulus and creep resistance of the elastic matrix, the coefficient of thermal expansion Small, lightweight, and excellent in electrical performance, it is suitable for advanced insulating paper and printed circuit boards with high dimensional stability. Lightweight high-density components of aramid pulp can be used for satellite communication lines, high-speed transmission circuits, etc. product.
3.2 Rubber products are combined with rubber by aramid pulp and rubber. The elasticity of rubber can be combined with the strength and rigidity of PPTA fiber to obtain a new and very useful engineering material, compared with long fiber reinforced materials. It can be used to manufacture engineering parts with complicated shapes and structures, and can also be used in the extrusion and transfer molding process widely used in the rubber industry. It does not need to make major changes to the existing rubber processing equipment, which can greatly simplify the production process of rubber products. Improve automation and continuity of production and reduce product costs. Its application in rubber products mainly has the following aspects.
Manufacture of rubber hoses. Aramid pulp fiber rubber compound can be directly extruded into various medium and low pressure pipes, which greatly simplifies the production process and reduces the product cost. Even in high pressure hoses, the number of braid layers and the weaving density can be reduced. It has economic significance.
Manufacturing power conveyors, conveyor belts and tank tracks. The aramid cord can be used to provide its main tensile strength, and the high modulus aramid pulp fiber imparts rigidity perpendicular to the cord direction, improves its longitudinal flexibility, and improves the friction between the belt and the pulley.
Manufacture of various seals. The addition of aramid pulp can significantly improve the dimensional stability, temperature resistance, pressure resistance and creep resistance of the rubber matrix, thereby improving the sealing performance of the seal and prolonging its service life.
Manufacture of tires and bicycle tires. It can greatly improve the tear resistance of the off-road tires, reduce the crack propagation rate and fatigue heat generation. Improve wear resistance and extend service life. Can be used as an ablative insulating material for missiles and rockets. Since the aramid has the properties of non-melting and carbonization at high temperature, the presence of a small amount of aramid pulp fiber can improve the ablation performance of the rubber matrix.
Various rubber rollers are manufactured to maintain the dimensional stability of the rubber roller.
3.3 As an alternative to asbestos fiber Asbestos is an inorganic fiber with many excellent properties, such as high tensile strength, toughness, heat resistance, chemical resistance, dimensional stability and stable friction properties. Used in industrial fields such as friction materials, sealing materials, fillers, etc., but its use can cause public hazards, making people have to work hard to find alternatives. Scientists from DuPont of the United States have introduced aramid pulp fibers and studied them in terms of characteristics, cost and products. They believe that the high tensile strength, light weight, long-term wear resistance and stable friction properties of aramid pulp High heat resistance, can meet the requirements of friction materials for reinforcing fibers, and has good processing performance. 1kg aramid is equivalent to 20kg asbestos, and it has competitive potential in price, which is an ideal substitute. At present, aramid pulp fibers have been applied in brake linings, automobile clutches and brake pads.
Aramid pulp fibers can also replace asbestos fibers in the fields of sealing fillers, construction, and insulating materials.
3.4 Electronic and electrical In recent years, due to the development of the electrical and electronic industry, aerospace electronic computers and electrical components have become more and more demanding on the requirements of miniaturization, light weight, high capacity and high reliability of products, and traditional fiberglass. Reinforced plastic electrical insulation products have not been able to meet this requirement. Due to its excellent mechanical properties, electrical insulation properties, wave transmission properties and dimensional stability properties, aramid fibers have been used in the field of electrical and electronic industry for special printed circuit boards for surface mount technology (SMT) in microelectronic assembly technology. , airborne or spaceborne radar radomes, radar antenna feed functional structural components and moving electrical components and many other aspects.
Due to the development of microelectronic assembly technology, SMT appeared in the late 1970s, and it has become one of the most remarkable new technologies. The International Electron Assembly Society and the International Microelectronics Society have made SMT an important part in recent years. The topic to discuss. SMT adopts short-bow I-line or leadless substrate carrier and small-sized piece. Due to the harsh working conditions of the electronic device, SMT must use sealed ceramic carrier. After the ceramic carrier and the glass cloth printed circuit board are welded, several high and low temperatures are passed. It is highly susceptible to cracking caused by thermal stress. If aramid fiber laminated printed circuit board is used. Since the linear expansion coefficient of the aramid fiber is small, the linear expansion coefficient of the laminated substrate made of the resin can be adjusted, and matching with the ceramic carrier can effectively reduce the stress caused by the temperature change.
After multi-layer printing, the board (PBW) is a competitive board developed in recent years. One of the important features of such a multilayer circuit board structure, particularly a high density package structure, is that the circuit board's channels are the channels through which electrical signals enter the composite circuit board from the semiconductor substrate. The use of aramid fiber in such a circuit board can reduce the dielectric loss coefficient and dielectric constant, making this type of circuit board more suitable for high-speed circuit transmission. The negative linear expansion coefficient of the aramid fiber in the fiber direction can reduce the coefficient of linear expansion of the entire substrate and reduce the crack caused by the temperature change of the ceramic multilayer package circuit board. In addition, such a substrate is also about 20% lighter than a glass fiber composite substrate.
DuPont began selling PPTA composite printed circuit boards in 1982. They also used PPTA fabrics to make high-density leadless electronic substrate mounts. The support has high tensile strength and good dimensional stability, can effectively inhibit thermal expansion and delamination of the resin matrix and copper due to heat, and has excellent dimensional stability. Teijin also uses its PPTA fiber as leadless. The reinforcing material of the ceramic substrate carrier is made into a special printed circuit board for the electronic industry; compared with the conventional epoxy glass cloth laminate, the circuit board has good dimensional stability, low dielectric constant, and is more suitable for high-speed lines. Transmission also contributes to miniaturization and weight reduction of electronic devices. In addition, Japan Toyo Textile Co., Ltd. has developed a flexible printed circuit board with high dimensional stability and high moisture resistance. The circuit board is made of a poly(m-phenylenediamine) fiber nonwoven fabric reinforced epoxy matrix resin. Compared with DuPont's PPTA49 reinforced composite, it has lower water absorption and better machinability.
Teijin's products are mainly used to make printed circuit boards for lead-free mounting of ceramic substrates. At present, such printed circuit boards have risen to more than 50% of the same type of printed circuit boards.
In the past, the radome was generally made of glass fiber reinforced plastic and glass reinforced plastic honeycomb sandwich structure. Since the advent of aramid fiber, the work of developing radomes with aramid fibers and their fabric composites has been carried out at home and abroad. In addition to its high specific strength, high specific modulus and low density, aramid fiber also has good dielectric properties, electromagnetic wave resistance, corrosion resistance and UV resistance, so it is a high-quality material for manufacturing radomes. .
Since the dielectric constant of the aramid composite is lower than that of the FRP, the thin-walled radome is guaranteed to have the same stiffness. The required thickness of the aramid fiber composite is thinner than that of the glass fiber composite, and the electrical properties such as transmittance are superior to those of the FRP radome of the same structure. For the half-wavelength radome, since the elastic modulus of the aramid material is high and the dielectric constant is low, the wall thickness of the lower magnification can be used to achieve the necessary structural rigidity, and the electrical performance can be greatly improved. For the sandwich radome, aramid paper honeycomb core material can be used to form a honeycomb layer structure.
In addition, due to its excellent mechanical properties and dielectric properties, aramid fibers and their fabric composites are also used in the manufacture of structural components and radar antennas, waveguides, feeds and chassis frames. .
Aramid chopped fibers have excellent wear and non-abrasive surface characteristics. It can be used to make brakes (brakes), clutches, circuit breakers, etc. for use in electrical and electronic equipment.
Japan's Unika company's meta-aramid pulp contains nearly 50% glass fiber, and the aramid paper made from this pulp can be easily impregnated with insulating varnish and used as the inner layer of insulating paper; the company's aromatic amine The combination of fiber and carbon fiber has good processability, heat resistance and semiconductivity, and can be used as a material for reducing electric field in high voltage devices. Recently, the company has developed synthetic pulps that are excellent in heat resistance, flame retardancy, and electrical insulation. It has been applied to heat-resistant motor insulation materials such as vehicle transformers, various generators and motors.
Due to the development of the European high-speed rail transportation network, the sales volume of PPTA fiber produced by DuPont in the United States continued to increase in the 1990s, mainly used for electrical insulation materials in generators and transformers, using the high specific strength of aramid fiber, DuPont PPTA fibers are also used in deep sea cables and pipe cables.
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